1. Field
Certain embodiments of the invention relate generally to communication systems, and more particularly, to wide area cellular communication systems such as Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), High Speed Packet Access (HSPA), and Third Generation Partnership Project (3GPP) Long-Term Evolution (LTE).
2. Description of the Related Art
Currently, there are efforts to design a local area radio system to complement existing cellular wide area systems, such as GSM, UMTS, HSPA, and LTE. Unlike a wide area cellular system, a local area system can utilize a license-exempt spectrum, or white spaces, to take advantage of additional bandwidth. In addition, a local area system can offer an efficient device-to-device operation mode to establish ad-hoc networks.
A radio interface of this local area system is derived from state-of-the-art radio designed in 3GPP LTE-Advanced (LTE-A) standardization. However, uncoordinated deployments and dynamic time division duplex (TDD) switching points are likely to be used in the local area system. The use of uncoordinated deployments and dynamic TDD switching points can result in high-interference scenarios, where there is an increased likelihood of data packet transmission collisions, because of simultaneous data packet transmissions. For example, interference can be high for data packet transmissions, in situations such as terminals in cell edges or uncoordinated deployments.
One particular interference scenario is identified as a “hidden node problem.” A hidden node in a cellular wide area system (or local area system) refers to a node of the system that is out of range of another node or a collection of nodes, where a node is a device connected to the cellular system (such as a user equipment (UE)). For example, node A may reside at a far edge of an access point's (AP's) range, and node B may reside at an opposite edge of the AP's range, where an AP is a device that allows a node to connect to the cellular system. For example, an AP may be an evolved Node B (eNB). Both node A and node B can sense the presence of, and transmit packets to, the AP. However, node A cannot sense the presence of node B, as node B is outside of node A's range, and visa-versa. Thus, node B is hidden to node A, and visa-versa. In the hidden node problem, nodes A and B simultaneously transmit packets to the AP because nodes A and B cannot sense each other, and thus, cannot determine that the other node is also transmitting a packet to the AP. The simultaneous transmission of packets to the AP causes collisions, which scrambles data packets.
In IEEE 802.11 standard, a Request-to-Send (RTS)/Clear-to-Send (CTS) protocol is an optional feature that attempts to solve the hidden node problem. According to the RTS/CTS protocol, an origination node that desires to transmit a packet to a destination node first transmits an RTS packet. The destination node replies with a CTS packet. Any other node that receives the RTS packet or CTS packet is required to refrain from transmitting packets until the transmission between the origination node and destination node has completed. In effect, the RTS/CTS protocol allows a node to reserve a channel in order to transmit data packets. Thus, the RTS/CTS protocol solves the hidden node problem, and provides for essentially interference-free transmissions. While RTS/CTS protocol is an example of a protocol that utilizes a handshake mechanism in order to transmit packets, other protocols exist as well that utilize a handshake mechanism.
FIG. 1A illustrates an example of a conventional RTS/CTS protocol exchange for a channel reservation. As illustrated in FIG. 1A, cellular system 100 includes APs A and B, and UE1 and UE2. AP A is located at the center of cell 101, and AP B is located at the center of cell 102. UE1 is a cell-edge user, as UE1 is located near the edge of cell 101, while still being within cell 101, and near the edge of cell 102. As illustrated in FIG. 1A, in accordance with the RTS/CTS protocol, before AP A and UE1 transmit data packets to one another, UE1 transmits a CTS packet to AP B, which is in range of UE1. Upon receiving the CTS packet, AP B refrains from transmitting to UE2, which allows AP A and UE1 to complete their transmission. Thus, a cell-edge user has an advantage of an essentially interference-free channel, leading to a small probability of transmission outage. This property makes the RTS/CTS protocol attractive for a multi-cell environment with a quality-of-service (QoS) target where a minimum guaranteed data rate is supported.
However, there can be several shortcomings in the application of the RTS/CTS protocol, particularly in conjunction with a carrier sense multiple access (CSMA) protocol, which can result in rate degradation in the system. First, the RTS/CTS protocol can lead to an inefficient utilization of resources, since channels cannot be reused between terminals, where the data packet transmission can tolerate a level of interference. In other words, the RTS/CTS protocol assumes that all data packet transmissions require an interference-free environment to be successful. If this is not the case, then data packet transmissions that would otherwise be successful are not allowed by the RTS/CTS protocol.
FIG. 1B illustrates an example of a direct application of the conventional RTS/CTS protocol, where data packet transmissions are inhibited, even though the transmissions would otherwise be successful. As illustrated in FIG. 1B, cellular system 110 includes APs A and B, and UE1 and UE2. AP A is located at the center of cell 111, and AP B is located at the center of cell 112. In FIG. 1B, both UE1 and UE2 simultaneously receive two RTS packets. According to the RTS/CTS protocol, UE1 fails to acknowledge AP A, and UE2 fails to acknowledge AP B, and thus, data packet communication is inhibited. However, in this scenario, a reasonable data rate between AP A and UE1, and a reasonable data rate between AP B and UE2, could be achieved using techniques such as link adaptation, despite the presence of interference. For example, a cellular system, such as a 3GPP LTE cellular system, can employ a link adaptation mechanism to adjust a data rate for each UE in such a way that some level of interference can be tolerated in the system, and thus, resources can be reused efficiently. However, because the RTS/CTS protocol assumes that all data packet transmissions require an interference-free environment to be successful, such efficient resource use is lost.
Second, the RTS/CTS protocol does not prevent all scenarios where a node interferes with a neighboring transmission and causes packets to collide. FIG. 2 illustrates examples of possible types of collisions that can occur with a conventional RTS/CTS protocol. In sequence 200, AP A transmits packet RTS (A) UE A, and AP B simultaneously transmit packet RTS (B) to user UE A. Because packet RTS (A) and packet RTS (B) arrive at UE A simultaneously, the packets collide, and UE A cannot decode the contents of the packet. This is known as RTS collision. Due to RTS collision, UE A does not transmit packet CTS (A) to AP A or AP B. Because AP A does not receive packet CTS (A), AP A fails to transmit data packet Tx A to UE A. Furthermore, due to the collision of packets RTS (A) and RTS (B), UE A does not forward packet RTS (A) to UE B. Because UE B does not receive packet RTS (A), UE B transmits packet CTS (B) to AP B. Upon receiving packet CTS (B), AP B transmits data packet Tx B to UE B.
Furthermore, in sequence 210, AP A transmits a packet RTS (A), and AP B transmits a packet RTS (B). UE A receives packet RTS (A), but does not receive packet RTS (B) due to packet loss (such as signal degradation over the system, an oversaturated channel, a corruption of the packet RTS (B) causing the packet to be rejected in-transit, or a underlying hardware problem). Because UE A only receives packet RTS (A), and does not receive packet RTS (B), UE A transmits packet CTS (A) to APs A and B. Furthermore, because UE B receives packet RTS (B), UE B transmits packet CTS (B) to AP B. In this case, there is a collision between packet CTS (A) and packet CTS (B) at AP B, and thus, AP B does not transmit data packet Tx B.